Laser ranging and detection systems are known in the art and generally consist of a laser light source coupled to an optical transmission system for illuminating a target. A photodetector receives backscattered incoherent light from the target and provides a signal indicative thereof to a ranging processor. The transmission/receiving optical systems often share a common telescope to meet design and cost requirements.
Conventional methods for separation of an outgoing coherent laser beam and received, backscattered, incoherent light, employ either a partially transmissive/reflective beam splitter or polarizer systems. Both transmissive/reflective beam splitters and polarizer systems cause a loss of receiver efficiency due to light losses. Ordinarily, these efficiency losses are compensated by use of a more powerful laser or a larger telescope.
Beam separators are used in laser radar systems, laser doppler velocimeters, photoluminescence sensors and other devices which employ a laser beam to illuminate a target media. Each of those systems requires a means for separating the outgoing laser beam from returned reflection light. In U.S. Pat. No. 3,968,362 to Mocker, a roof prism reflector separates the beam paths of an illuminating source and a receiving detector. The roof prism reflector only reflects only up to half of the return light and feeds it to a detector.
In U.S. Pat. No. 5,098,185 to Watanabe et al., apparatus for beam splitting includes a parabolic reflector that includes an aperture through which a scanned laser beam is directed. The parabolic reflector is oriented at such an angle that a return beam is reflected off-axis to a receiver. The transmitted laser beam is scanned through the aperture in the reflector, requiring that the aperture be sufficiently large to allow passage of the scanned beam, without obstruction. The Watanabe et al. system employs a laser beam collimator which expands the beam size and maintains its cross section at a constant diameter over an extended distance. In practice, "collimated" beams typically have diameters approximately 1 millimeter or greater, otherwise, diffraction expands the beam diameter over a distance as short as one meter or less.
In U.S. Pat. No. 5,141,312 to Thompson et al., a photoluminescence sensor is illustrated that passes an incident laser beam through an aperture in a concave mirror to a microscope objective system that includes primary and secondary mirrors. The return beam is reflected off-axis by the concave mirror to a detector. Because Thompson et al. employ no means for reducing the cross section of the laser beam, a relatively large diameter aperture is required in the concave mirror for the outgoing beam, thereby reducing the amount of reflected light that can be directed to the detector.
Accordingly, it is an object of this invention to provide a laser ranging and detection system with an improved beam separation apparatus.
It is a further object of this invention to provide a laser ranging and detection system with a beam separator that enables greater than 90 percent of received incoherent light to be directed to a light detection system.
It is yet another object of this invention to provide a laser ranging and detection system with an improved geometric beam separator that enables the system to employ a single, aligned telescope with simple, inexpensive optics.